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5.1 Parts of Air Brake System (continued from previous page)

Brake Pedal

You apply the brakes by pushing down the brake pedal (also called the “foot valve” or “treadle valve”). Pushing the pedal down harder applies more air pressure. Letting up on the brake pedal reduces the air pressure and releases the brakes. Releasing the brakes lets some compressed air go out of the system, so the air pressure in the tanks is reduced. It must be made up by the air compressor. Pressing and releasing the pedal unnecessarily can let air out faster than the compressor can replace it. If the pressure gets too low, the brakes will not work.

Foundation Brakes

Foundation brakes are used at each wheel. The most common type is the S-cam drum brake (see Figure 5-2 below). The parts of the brake are discussed below:

Brake drums, shoes, linings:

Brake drums are located on each end of the vehicleʼs axles. The wheels are bolted to the drums. The braking mechanism is inside the drum. To stop, the brake shoes and linings are pushed against the inside of the drum. This causes friction, which slows the vehicle (and creates heat). The heat a drum can take without damage depends on how hard and how long the brakes are used. Too much heat can make the brakes stop working.

S-cam brakes:

When you push the brake pedal, air is let into each brake chamber (see Figure 5-2). Air pressure pushes the rod out, moving the slack adjuster, thus twisting the brake cam shaft. This turns the S-cam (named because it is shaped like the letter S). The S-cam forces the brake shoes away from one another and presses them against the inside of the brake drum. When you release the brake pedal, the S-cam rotates back, and a spring pulls the brake shoes away from the drum, letting the wheels roll freely again.

Figure 5-2
S-cam Air Brake

Wedge brakes:

In these types of brakes, the brake chamber push rod pushes a wedge directly between the ends of two brake shoes. This shoves them apart and against the inside of the brake drum. Wedge brakes may have a single brake chamber or two brake chambers, pushing wedges in at both ends of the brake shoes. Wedge-type brakes may be self-adjusting or may require manual adjustment.

Disc brakes:

In air-operated disc brakes, air pressure acts on a brake chamber and slack adjuster, like S-cam brakes. But instead of the S-cam, a “power screw” is used. The pressure of the brake chamber on the slack adjuster turns the power screw. The power screw clamps the disc or rotor between the brake lining pads of a caliper, similar to a large C-clamp.

The manual adjustment of automatic slack adjusters is dangerous because it gives the vehicle operator a false sense of security about the effectiveness of the braking system.

Wedge brakes and disc brakes are less common than S-cam brakes.

Get ready for some studying! You need to know that "Foundation Brakes" refer to the brakes located at each individual wheel. There are many different types of foundation brakes, but the most common one is the S-Cam drum shown in figure 5-2. For the pre-trip exam, you'll need to know all the parts of an S-Cam brake system shown in that figure. For the written exam, you'll need to know the different types of foundation brakes and how they work, but pay particular attention to the S-Cam brake as you're more likely to be asked questions about that type of foundation brake.
Earlier in this course, we discussed a term used to describe when brakes heat up and lose their effectiveness. do you remember that term? It's called "Brake Fade" and is a term you need to know.
For the pre-trip exam, you'll need to have a full understanding of the S-Cam brake, including memorizing all of the main parts of the system. But for the written exam, you only need to have a general understanding of how it works.
It is pretty unlikely that you'll drive any commercial vehicle with anything other than S-Cam brakes. But you still need to have a general understanding of each type of brake for the written exam.

CDL:

Commercial Driver's License (CDL)

A CDL is required to drive any of the following vehicles:

  • Any combination of vehicles with a gross combined weight rating (GCWR) of 26,001 or more pounds, providing the gross vehicle weight rating (GVWR) of the vehicle being towed is in excess of 10,000 pounds.
  • Any single vehicle with a GVWR of 26,001 or more pounds, or any such vehicle towing another not in excess of 10,000 pounds.
  • Any vehicle, regardless of size, designed to transport 16 or more persons, including the driver.
  • Any vehicle required by federal regulations to be placarded while transporting hazardous materials.

Review Questions - Click On The Picture To Begin...

What happens when the brake drums exceed their designed temperature?
  • Too much heat can make the brakes stop working
  • Too much heat will cause the brakes to "catch" and potentially lock up the brakes
  • Too much heat will activate the water-cooled emergency system
  • Too much heat will force the emergency brakes to engage

Quote From The CDL Manual:

Too much heat can make the brakes stop working.

TruckingTruth's Advice:

As the brake drums heat up, they become less and less effective. This is a process known as "brake fade" which is a very popular term in the trucking industry. After a while, they will become totally useless. Brake Fade is the #1 cause of brake failure on steep downgrades.

Next
What are foundation brakes?
  • Foundation brakes are also known as the emergency brakes
  • Foundation brakes are the brakes used when depressing the brake pedal
  • Foundation brakes are used at each wheel and make up the individual components of the brake system
  • Foundation brakes are used for long-term parking when it is believed the air pressure will deplete over time

Quote From The CDL Manual:

Foundation brakes are used at each wheel. The most common type is the S-cam drum brake.

TruckingTruth's Advice:

While you may not be asked about the specific term "foundation brakes" you will absolutely need to know the different parts of an S-Cam brake drum. Be sure to study those parts and know what they do. This will be important not only for your written exam, but also your pre-trip exam.

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Next
What accurately describes an S-Cam?
  • When you release the brake pedal, the S-cam rotates back, and a spring pulls the brake shoes away from the drum, letting the wheels roll freely again
  • Forces the brake shoes away from one another and presses them against the inside of the brake drum
  • All of these answers are accurate
  • It is called an S-Cam because it's in the shape of an S

Quote From The CDL Manual:

S-cam brakes: When you push the brake pedal, air is let into each brake chamber (see Figure 5-2). Air pressure pushes the rod out, moving the slack adjuster, thus twisting the brake cam shaft. This turns the S-cam (named because it is shaped like the letter S). The S-cam forces the brake shoes away from one another and presses them against the inside of the brake drum. When you release the brake pedal, the S-cam rotates back, and a spring pulls the brake shoes away from the drum, letting the wheels roll freely again.

TruckingTruth's Advice:

You need to study the different parts of the S-Cam air brake system. Be sure to know the major parts and understand how it works.

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Next
The following statements accurately describe Wedge Brakes, except:
  • All wedge-type brakes are self-adjusting
  • The brake chamber push rod pushes a wedge directly between the ends of two brake shoes
  • All of these answers are accurate
  • Wedge brakes may have either a single brake chamber or two brake chambers

Quote From The CDL Manual:

Wedge brakes: In these types of brakes, the brake chamber push rod pushes a wedge directly between the ends of two brake shoes. This shoves them apart and against the inside of the brake drum. Wedge brakes may have a single brake chamber or two brake chambers, pushing wedges in at both ends of the brake shoes. Wedge-type brakes may be self-adjusting or may require manual adjustment.

TruckingTruth's Advice:

Wedge brakes are derived from old technology and are very rarely used in modern commercial vehicles. However, you still may be asked a question or two about wedge brakes on your written exam.

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Next
Which statement best describes Disc Brakes?
  • When driving, powerful springs are held back by air pressure and if the air pressure is removed, the springs put on the brakes
  • Air pressure pushes a push rod out, moving the slack adjuster, thus twisting the brake cam shaft
  • The brake chamber push rod pushes a wedge directly between the ends of two brake shoes
  • A power screw clamps the disc or rotor between the brake lining pads of a caliper, similar to a large C-clamp

Quote From The CDL Manual:

Disc brakes: In air-operated disc brakes, air pressure acts on a brake chamber and slack adjuster, like S-cam brakes. But instead of the S-cam, a "power screw" is used. The pressure of the brake chamber on the slack adjuster turns the power screw. The power screw clamps the disc or rotor between the brake lining pads of a caliper, similar to a large C-clamp.

TruckingTruth's Advice:

While it is very likely you will only drive a vehicle with S-Cam brakes, you should know the difference between S-Cam brakes, wedge brakes, and disc brakes.

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